Restarting your estimate will delete your workbook and therefore all of the data you've entered so be careful about restarting. That being said, you do have to restart to accomplish any of the following:
You want to change your operating costs
You want to edit your product or service mix
If you are very careful about version control, you may find it helpful to make a copy of your workbook which then you can paste back into a new workbook after you re-estimate.
To restart your estimate navigate to the Measure tab in your portal, scroll to the bottom of the page, and click Restart Measurement.